Mechanical air classifier



March 1, 1966 G. D. HUMPHREYS MECHANICAL AIR CLASSIFIER 4 Sheets-Sheet 1Filed 061;. 18, 1961 March 1, 1966 HUMPHREYS 3,237,766

MECHANICAL AIR CLASSIFIER Filed Oct. 18, 1961 4 Sheets-Sheet 2 March 1,1966 HUMPHREYS 3,237,766

MECHANICAL AIR CLASSIFIER Filed Oct. 18, 1961 4 Sheets-Sheet S March 1,1966 HUMPHREYS 3,237,766

MECHANICAL AIR CLASSIFIER Filed Oct. 18, 1961 4 Sheets-Sheet 4 MW BY 5mgUnited States atent 3,237,766 MECHANICAL AIR CLASSIFIER Geoffrey DanielHumphreys, Hartley, Kent, England, assignor to The Associated PortlandCement Manufactnrers Limited, London, England, a company of the UnitedKingdom Filed oct. 18, 1961, Ser. No. 145,810 Claims priority,application Great Britain, Oct. 21, 1960, 36,282/60 6 Claims. (Cl.209-139) This invention relates to classifying machines.

Air or gas classifiers operate on the principle of subjecting airborneparticles to centrifugal forces which act in a direction different fromthe direction of flow of the air or other working fluid.

In the standard type of classifier, the material to be classified is fedinto the innermost of two concentric cones, air being drawn upwardlythrough the inner cone by an inbuilt fan at the top, thereby drawing airup through the inner cone and down through the annular space separatingthe cones. Material fed in at the top falls onto a rapidly revolvingfeed plate, by which it is thrown outwardly in a thin layer by thecentrifugal force and some of the coarsest material falls to the bottomof the inner cone, most of the material being lifted by the fan into theclassifying zone. The material passing through the classifying zoneproceeds through the fan into the outer cone, where it separates fromthe carrying air as in a cyclone.

In the classifying zone the particles of the material are subjected to africtional drag force, which is Caused by the flow of air induced by thefan, and a centrifugal force caused by the rotation of the classifyingzone. In the more successful commercial classifiers these two forces areinduced to act in substantially opposite directions, but in manyclassifiers these forces are at right angles to each other. If ahomogeneous material (i.e. the density of each particle is independentof the particle size) is carried by an air stream of a given velocityand is caused to rotate at a given velocity and radius about the centreof a classifier, there is a critical size of particle, for which thecentrifugal and drag forces are equal.

In practice it is found that the drag force exerted on a particleincreases rapidly as the particle travels inwards, whilst thecentrifugal force exerted on the particle decreases progressively withthe result that only a small proportion at the periphery of theclassifying zone is effective.

To overcome this difiiculty, it has been proposed so to design theclassifier that the vertical height within the classifying zoneincreases as the radius becomes less i.e. it is greater at the centrethan at the periphery. With this construction it becomes possible toproduce a design in which the ratio of drag and centrifugal force actingon a particular particle is independent of the radius of rotation i.e.its position in the classifying zone with the result that at all pointsacross the classifying zone, there is virtually a proper balance betweendrag and centrifugal force acting on the particles. However hitherto,practical difficulties in constructing a classifier have resulted byreason of the tendency of the cut size particles to remain stationary ata particular section without moving either inwardly or outwardly.

According to this invention, in the operation of a classifier of thesingle or double cone type, the cross sec tion of the classifying zoneis greater at or adajcent the centre than at its periphery so that atall points the drag set up by the air or gas flow substantially balancesthe centrifugal force acting on a particle of the size at which a cut isdesired and the direction of flow of the air or gas, prior to entry intothe classifying zone, is approximately parallel to the axis of rotationin order to ensure inside the classifying zone a change in rate of flowextending vertically across it whereby cut particles, which aresuspended in the air stream, will be released and rejected, when, due togravity, they move downwards into that part of the stream where thedrag, due to reduced velocity, no longer balances the centrifugal force.

The invention is illustrated in the accompanying drawings in which:

FIGURE 1 is a diagrammatic sectional view of a mechanical air classifierhaving a standard, i.e. prior art type of classifying zone,

FIGURE 2 is a part sectional view wherein the classifying zone is inaccordance with a preferred embodiment of the invention,

FIGURE 3 is a sectional view of a classifier for use in an air sweptgrinding circuit having a classifying zone in accordance with theinvention,

FIGURE 4 is a sectional view of a classifier similar to that shown inFIGURE 3 but provided with means for adjustment of the depth of theclassifying zone.

In the drawings, there is shown in FIGURE 1 a classifying machine ofpreviously known construction having inner and outer cones, respectivelyindicated at 1 and 2, circulation of air being by means of a fan 3 fastupon a central shaft 4 having a drive pulley 5. The inner cone 1includes an upper portion 1, the two parts 1 and 1' having securedtherebetween an apertured partition plate 6 in the form of radial orskew vanes so as to provide circumferentially spaced circulationpassages for the return circuit of the air flow set up by the fan 3, thepath taken by the air flow being shown by the arrows a. Solid materialto be classified is fed to the classifier through a feed chute 8 fromwhence it falls onto a revolving feed plate 9 fast on the shaft 4 and bymeans of which the material is distributed across the air stream.

The material received by the plate 9 is thrown outward in a thin streamby centrifugal force and some of the coarsest material falls to thebottom of the inner cone. Most of the material however is entrained inthe air-stream and lifted by the fan 3 into what is known as theclassifying zone 7. The material passing the classifying zone 7 proceedsthrough the fan 3 into the outer cone 2, where it separates from thecarrying air as in a cyclone. After this separation the air isrecirculated by passage through the vanes 6 into the inner cone 1 thuscompleting its cycle. The heavier material rejected in the classifyingzone 7 falls to the bottom of the inner cone 1.

In existing designs of classifier the classifying zone is made up ofradial blades 10 carried by a lower shroud plate 11, which assemblyrotates with the fan shaft 4.

Referring now to FIGURE 2, which shows a casing having inner and outercones 1 and 2, as in FIGURE 1, the classifying zone according to thisinvention is bounded by a continuous annular upper shroud plate 12 and atable or lower shroud plate 13, secured between which are a series ofradially arranged blades or walls 14 and it will be seen that the shapeof the top shroud 12 is such that the cross-sectional area of the space15 between adjacent blades or walls 14 increases towards the centre. Theupper part 1 of the inner cone 1 serves as a cylindrical guard ring, apart of which surrounds the classifying zone, the arrangement being suchthat air moved by the fan 3 passes between the guard ring and theperiphery of the table 13 and thence through the classifying zone towardthe axis of the table 13 and through the central outlet delineated bythe radially inner edge of the annular shroud plate 12. The guard ringis formed with an inwardly directed lip 1" and the shroud plate 12 isformed with a portion 12" which overlaps the lip 1 and cooperatestherewith to cause most of the air to enter the classifying zone.

As a result of the increase in cross section of the space 15 toward thecentre, the velocity of the air passing through the classifying zoneprovided by the rotor 12, 13, 14 decreases in such a way that thefrictional drag force exerted on a particular particle, which istraversing the classifying zone, decreases at the same rate as thecentrifugal force acting on the particle. Thus, for any given speed ofrotation of the hollow shaft 4, through which particulate material to beclassified is fed, and setting of the air flow control damper 16, whichis adjustable, the ratio of the drag and centrifugal forces on aparticle in the classifying zone can be made virtually constant andindependent of the position of the particle in the zone.

As stated above, when operating the classifier, there is a tendency forparticles of the size at which separation is required, to remain in theclassifying zone and neither to move radially inwards nor outwardsbecause the inward drag more or less exactly balances the outwardcentrifugal force acting on them.

It has been found that this difiiculty of removing the cut sizeparticles can be overcome by arranging for the direction of the airflow, prior to entering the outer peripheral opening in the rotor to theclassifying zone, to be in a direction, which is parallel, orapproximately parallel, to the axis of rotation of the shaft so that onentering the classifying zone, it is forced to move in a path which isat right angles to the plane of the zone.

It follows that, at the point of entry, the air flow takes a suddenchange of direction, as the result of which the inward velocity willvary vertically across the classifying zone throughout its entirelength. Thus the air flow, as represented by the arrows A, will be at amaximum and substantially exceed the rate of the air flow represented bythe arrows B immediately above the lower shroud plate 13.

As a result, cut size particles, which would otherwise tend to remain inposition in the classifying zone, and which, after a period of time,would block the zone completely, will fall under the action of gravityfrom the region where the air stream is at the maximum or higher rate offiow to a region where it is a minimum.

In moving from a region where the air flow is at a maximum rate to onewhere it is lower, it will be obvious that centrifugal force, whichremains constant for any given radius of rotation will cause theparticles to be rejected because the centrifugal force is no longerbalanced by the drag force set up by the air fiow, which becomessmaller.

The value of the ratio of the drag and centrifugal force for anyparticle will clearly depend on the particle size. For a particle abovea certain critical size the ratio of the drag to the centrifugal forcewill be less than unity, which results in the particle being rejected.This critical size may be increased or decreased by the opening orclosing respectively of the control damper 16. This control damperconsists of a sleeve surrounding the vanes 6 and supported by adjustingscrews 17, of which there would normally be three. These adjustingscrews are operated by handles 18 and may be linked together bysprockets 19 and a chain (not shown).

In the embodiment shown in FIGURE 2 the classifying zone is shown with afiat lower table or plate 13 and a curved upper shroud plate 12. Similarconditions in the classifying zone i.e. of balance between the inwardareodynamic drag force and centrifugal forces acting outwardly onparticles of critical size throughout the classifying zone may also beachieved in the case where the zone is bounded by top and bottom plates,either or both of which may be fiat, conical or curved.

It is also possible to utilise the principle of the invention in thedesign of classifiers for air swept grinding mills, where the control ofthe air velocity is by means of an external fan. In this case the speedof rotation of the classifying section may also be varied for controlpurposes in order to select the appropriate critical particle size.

A typical arrangement for a classifier of this type is shown in FIGURE3, in which the classifier comprises a conical chamber 20 having abottom inlet 21 adapted for connection to a grinding mill to receive theair or gas flow from the grinding mill (not shown). On passage throughthe classifying zone 22, oversize particles are removed and injectedinto the chamber 20 and thence through a return conduit 23. Theclassified particles on leaving the outflow duct 24 pass, by inductionof a fan (not shown) connected to the duct 24 externally of theclassifier, to a cyclone or other material collecting device, or to thefiring equipment in the case of a direct fired coal grinding circuit.

Referring now to FIGURE 4 there is shown a classifier similar to that ofFIGURE 3 but in which provision is made for adjustment of the depth ofthe classifying zone.

In this arrangement the driving shaft 30 is tubular and is fitted withan interior sleeve 31, having at its lower end a flange 32 supporting atits perimeter a number of rods 33, to the upper end of which rods issecured a ring 34.

The ring 34 provides a support for the curved upper plate 35 of theclassifying zone, while a lower plate 36 is provided with apertures 37to accommodate the rods 33. Radial slots 38 are also cut in the plate 35for the blades 14'.

Should it be necessary to adjust the height of the classifying zone 22,this can readily be done by raising or lowering the sleeve 31 and thusthe ring 34 carrying the plate 35. The provision of an adjustableprofile plate 35 enables the ratio of the drag to centrifugal force tobe varied so that it is constant throughout the Zone or eitherincreasing or decreasing as the material passes from the periphery tothe centre of the zone.

It has been found that with a constant ratio rejects which arereasonably free from fines will be obtained and a product which isreasonably free from coarse material. In some cases, however, it may bedesirable (by suitable adjustment of the profile) to produce fines thatare virtually free from coarse materials while allowing some finematerial to pass out of the classifier with the rejects or vice versa.

It should be understood that the invention is not confined to theparticular shape of the classifying zone shown in FIGURES 2-4 since theshape of the throat as determined by the upper shroud plate 12 or 35 maybe varied according to the material to be handled and the gas used. Theparticle size, particle shape and operating gas velocity must all betaken into account when determining the optimum shape of the classifyingzone in order that the frictional drag force exerted on the particles ofthe material traversing the classifying zone decreases at substantiallythe same rate as the centrifugal force thereon.

What is claimed is:

1. In and for -a classifier of the centrifugal type for particulatematerial and comprising a casing; a first rotary table rotatable aboutan axis central in said casing; feed means for delivering particulatematerial substantially axially on to said rotary table for subsequentejection therefrom at the periphery of said first table undercentrifugal force; centrifugal fan means above said first rotary tablefor causing a flow of air past the periphery of said first rotary table,the ejected particulate material being entrained in said flow of airadjacent the periphery of said first table; a rotor comprising a secondrotary table rotatable about said axis and located above said firstrotary table and below the centrifugal fan means, walls on said secondtable having one end substantially at the periphery of said second tableand extending a substantial distance from the periphery thereof towardthe axis thereof, and a shroud plate axially spaced above the surface ofthe second table to form a classifying zone therebetween for said airand entrained particulate material to flow therethrough in a centripetaldirection and to be rotated by said walls, said shroud plate being ofcontinuous annular construction to provide a peripheral opening servingas an inlet to said classifying zone for said flow of air with entrainedparticulate material, and as an outlet for particles rejected undercentrifugal force, and to provide a central outlet for air and entrainedparticulate material, said second table and said shroud plate beingshaped to provide a flow cross-section increasing toward said centraloutlet at a rate such that for a particle of given size the ratio of theinward radial aerodynamic drag force to the centrifugal force issubstantially the same throughout said classifying zone, and a guardring at least part of which surrounds the classifying zone and issubstantially cylindrical and coaxial with the second table to cause airand entrained particulate material to enter the periphery of theclassifying zone with different inward radial components of velocityover the depth of said peripheral inlet opening.

2. A classifier according to claim 1 in which said feed means comprisesan axially rotatable tube journalled in said casing and terminating atits lower end above the first rotary table, said tube having afiixedthereto the first and second rotary tables, and said fan means comprisesa plurality of fan blades afiixed to said tube.

3. A classifier according to claim 2 including a ring movable foreffecting relative adjustment of the axial spacing between said shroudplate and said second table.

4. A classifier as claimed in claim 2 in which the guard ring is formedwith an inwardly projecting lip and the shroud plate is formed with anoverlapping portion for cooperation therewith.

5. A classifier as claimed in claim 2 in which said walls are in theform of radial blades and the shroud plate is secured thereto.

6. In and for a classifier of the centrifugal type for particulatematerial and comprising a casing; a first rotary table rotatable aboutan axis central in said casing; feed means for delivering particulatematerial on to said first rotary table for subsequent ejection therefromunder centrifugal force and including an axial duct terminating 4 at itslower end above the first rotary table; external duct means connected tosaid casing for delivering air into said casing and discharging air fromsaid casing for thereby causing a fiow of air past the periphery of saidfirst rotary table, the ejected particulate material being entrained insaid fiow of air adjacent the periphery of said first table; a rotorcomprising a second rotary table rotatable about said axis and locatedabove said first rotary table, walls on said second table having one endsubstantially at the periphery of said second table and extending asubstantial distance from the periphery thereof toward the axis thereof,and a shroud plate axially spaced above the surface of the second tableto form a classifying zone therebetween for said air and entrainedparticulate material to flow therethrough in a centripetal direction andto be rotated by said walls, said shroud plate being of continuousannular construction to provide a peripheral opening serving as an inletto said classifying zone for said flow of air with entrained particulatematerial, and as an outlet for particles rejected under centrifugalforce, and to provide a central outlet for air and entrained particulatematerial, said second table and said shroud plate being shaped toprovide a flow cross-section increasing toward said central outlet at arate such that for a particle of given size the ratio of the inwardradial aerodynamic drag force to the centrifugal force is substantiallythe same throughout said classifying zone, and a guard ring at leastpart of which surrounds the classifying zone and is substantiallycylindrical and coaxial with the second table whereby air and entrainedparticulate material enter the periphery of the classifying zone Withdifferent inward radial components of velocity over the depth of saidperipheral inlet opening.

References Cited by the Examiner UNITED STATES PATENTS 2,276,761 3/1942Carey 209-444 2,286,987 6/ 1942 Sturtevant 209-139 2,616,563 11/1952Hebb 20914'4 2,796,173 6/1957 Payne 209144 2,899,139 8/1959 Hardinge241-19 2,922,520 1/ 1960 Gustavsson 209-144 FRANK W. LUTTER, PrimaryExaminer.

HARRY B. THORNTON, HERBERT L. MARTIN,

ROBERT A. OLEARY, Examiner.

1. IN AND FOR A CLASSIFIER OF THE CENTRIFUGAL TYPE FOR PARTICULATEMATERIAL AND COMPRISING A CASING; A FIRST ROTARY TABLE ROTATABLE ABOUTAN AXIS CENTRAL IN SAID CASING; FEED MEANS FOR DELIVERING PARTICULATEMATERIAL SUBSTANTIALLY AXIALLY ON TO SAID ROTARY TABLE FOR SUBSEQUENTEJECTION THEREFROM AT THE PERIPHERY OF SAID FIRST TABLE UNDERCENTRIFUGAL FORCE; CENTRIFUGAL FAN MEANS ABOVE SAID FIRST ROTARY TABLEFOR CAUSING A FLOW OF AIR PAST THE PERIPHERY OF SAID FIRST ROTARY TABLE,THE EJECTED PARTICULATE MATERIAL BEING ENTRAINED IN SAID FLOW OF AIRADJACENT THE PERIPHERY OF SAID FIRST TABLE; A ROTOR COMPRISING A SECONDROTARY TABLE ROTATABLE ABOUT SAID AXIS AND LOCATED ABOVE SAID FIRSTROTARY TABLE AND BELOW THE CENTRIFUGAL FAN MEANS, WALLS ON SAID SECONDTABLE HAVING ONE END SUBSTANTIALLY AT THE PERIPHERY OF SAID SECOND TABLEAND EXTENDING A SUBSTANTIAL DISTANCE FROM THE PERIPHERY THEREOF TOWARDTHE AXIS THEREOF, AND A SHROUD PLATE AXIALLY SPACED ABOVE THE SURFACE OFTHE SECOND TABLE TO FORM A CLASSIFYING ZONE THEREBETWEEN FOR SAID AIRAND ENTRAINED PARTICULATE MATERIAL TO FLOW THERETHROUGH IN A CENTRIPETALDIRECTION AND TO BE ROTATED BY SAID WALLS, SAID SHROUD PLATE BEING OFCONTINUOUS ANNULAR CONSTRUCTION TO PROVIDE A PERIPHERAL OPENING SERVINGAS AN INLET TO SAID CLASSIFYING ZONE FOR SAID FLOW OF AIR WITH ENTRAINEDPARTICULATE MATERIAL, AND AS AN OUTLET FOR PARTICLES REJECTED UNDERCENTRIFUGAL FORCE, AND TO PROVIDE A CENTRAL OUTLET FOR AIR AND ENTRAINEDPARTICULATE MATERIAL, SAID SECOND TABLE AND SAID SHROUD PLATE BEINGSHAPED TO PROVIDE A FLOW CROSS-SECTION INCREASING TOWARD SAID CENTRALOUTLET AT A RATE SUCH THAT FOR A PARTICLE OF GIVEN SIZE THE RATIO OF THEINWARD RADIAL AERODYNAMIC DRAG FORCE TO THE CENTRIFUGAL FORCE ISSUBSTANTIALLY THE SAME THROUGHOUT SAID CLASSIFYING ZONE, AND A GUARDRING AT LEAST PART OF WHICH SURROUNDS THE CLASSIFYING ZONE AND ISSUBSTANTIALLY CYLINDRICAL AND COAXIAL WITH THE SECOND TABLE TO CAUSE AIRAND ENTRAINED PARTICULATE MATERIAL TO ENTER THE PERIPHERY OF THECLASSIFYING ZONE WITH DIFFERENT INWARD RADIAL COMPONENTS OF VELOCITYOVER THE DEPTH OF SAID PERIPHERAL INLET OPENING.